1. Field of the Invention
The present invention relates to a loudspeaker including a magnetic circuit using a plurality of magnets, and a voice coil assembly using bobbins and coils. More particularly, the present invention relates to a flat thin loudspeaker with a small overall height.
2. Description of the Related Art
Some electrodynamic loudspeakers for converting audio signals to sound employ flat diaphragms in order to realize a loudspeaker having a small overall height including the magnetic circuit. With a flat diaphragm, as compared with a cone diaphragm, the overall height of the loudspeaker will be small, but the loudspeaker is likely to have insufficient rigidity for vibrating as an integral unit without divided vibrations. In view of this, a conventional type of loudspeaker includes a bobbin fixed to a flat diaphragm and including a plurality of voice coils, wherein the flat diaphragm is driven by the plurality of voice coils, in an attempt to suppress divided vibrations. Another conventional type of loudspeaker is a flat thin loudspeaker with a small overall height, including a flat diaphragm provided with a plurality of coils and a magnetic circuit with a plurality of magnets corresponding to the coils.
Japanese Laid-Open Patent Publication No. 60-18098 discloses a flat thin loudspeaker with a small overall height, in which a plurality of coils are wound along deep grooves of a flat diaphragm having protrusions/depressions so that the coils are placed in magnetic gaps formed between a plurality of magnets. The coils are obtained by separating, in the separation step, a coil wound around in a racetrack pattern, and are then attached and bonded to the flat diaphragm. Japanese Laid-Open Patent Publication No. 63-299500 discloses a flat loudspeaker using a bobbin obtained by vacuum-forming a thin plastic plate, with coils being inserted along slits in the bobbin. In such a loudspeaker, coils are placed in magnetic gaps in the magnetic circuit, which are the places with the highest magnetic flux density, thereby increasing the efficiency of a flat loudspeaker. However, in order to realize desirable sound reproduction by a flat thin loudspeaker with a small overall height, a bobbin accommodating a plurality of coils placed in magnetic gaps needs to have a light weight and a high rigidity.
Japanese Laid-Open Utility Model Publication No. 55-88590 discloses a flat-plate loudspeaker using a number of rectangular voice coils that are joined together along the longitudinal and lateral sides thereof into a voice coil cluster, which is attached to a flat-plate diaphragm and driven by a magnetic circuit. As shown in FIG. 3 of this publication, the loudspeaker includes a plurality of magnetic circuits each having a magnet interposed between one long sides of a pair of strip-shaped yokes, with each side portion of each voice coil being inserted between the other long sides of the strip-shaped yokes of the magnetic circuits. This publication states that coils are wound around frames to form rectangular voice coils, which are then joined together along the longitudinal and lateral sides thereof into a voice coil cluster.
However, as seen in FIG. 2 of this publication, each voice coil of the conventional voice coil cluster is wound around the outside of a frame (equivalent to a bobbin), and adjacent ones of the bobbins forming the voice coil cluster cannot be attached closely together, and will rather be spaced apart from each other by the thickness of the wire diameter of the voice coils, even when adjacent voice coils are jointed together. If the bobbin rigidity is insufficient, bobbins may vibrate to cause divided vibrations, and adjacent bobbins may contact each other to produce an abnormal noise. Solving this problem will require the provision of an additional mass, e.g., an adhesive for connecting together the adjacent bobbins. This increases the total weight of the loudspeaker diaphragm, thereby reducing the reproduction efficiency of the loudspeaker.
Japanese Laid-Open Patent Publication No. 7-131892 discloses a voice coil obtained by winding a coil on the inner side of a bobbin, together with a method for reducing the number of winding steps required for winding a coil on the inner side of an annular bobbin.